Treatment of wood



e-iic 3R MUQQ Ethel ltialt'ub Mil-\KUH 'liUUM 2,395,311 TREATMENT OF WOOD John G. Woodhouse, Cragmere, andClarence D. BeILWiImington, DeL, assignors to E. I. du'

Pont de Nemours &-.- Company, Wilmington, DeL, a corporation of Delaware j I No Drawing. hpplication october'e, 1941,

Serial No. 413,654 7 2 Claims. (Cl. 117-147) This invention relates to the treatment of wood and more particularly to color control in chemical seasoning of Wood.

In the lumber industry, one of the. most serious problems has been that of checking and ,crack- 5' The quantities of urea employed may be varied ing of wood during its storage and drying after over a fairly wide rangesuch, for example, as is cutting of the timber. This checking and crackdisclosed in the process described in U. S. Serial ing of wood is due, primarily, to the shrinkage re- 6, new t 3 25%, wherein sulting from drying of the outer portions of green W od is treated with a solution of urea. Althou h wood at a more rapid rate than the interior pordescribed in 1 531 in the pplic tion referred to, tions. As a consequence of this uneven shrinkage, this par e l r t eatment calls for the use surface .strain's'and stresse a set up which of urea solutionsofsuch concentration that, at cause extensive cracking, checking or splitting of t e P r r empe tur employed the urea the green lumber, being most apparent at the solution is complete y r ed t the wood'surk' ends and along the surface of the lumber in the 1 5 f T s invention s n li to pp central portions of the flat grain cuts.

In the copending application S. N. 353,696, now U. syPatent 2,302,594, a method is disclosed for overcoming this checking and cracking which comprises, in summary, treating green or unseasoned wood with urea solutions of such concentration, at th particular temperatures employed, that at the wood surfaces the urea solution iscompletely saturated. In application of the process of Serial No. 353,696, orother methods of apply ing urea to wood, it has been found that the color changewhich ordinarily takes place when wood is air-dried or kiln dried is accentuated in some cases, particularly in kiln drying at high tem-' peratures. Some species of wood are much worse in this respect than others and also some species such as oak, when treated with urea and allowed to air season under exposed conditions develop irregular black-brownish areas that penetrate seasoning lumber with urea without formationof unnatural color in the dried wood.

It is an additional object of the present invention to provide a method for treating wood with urea in such a way that checking and cracking are inhibited and at the same time the wood may be bleached during drying operations.

According to the present invention, wood which is susceptible to cracking and checking during drying or seasoning and which is also susceptible as well as color change; by treatment, prior to dry-i ing, with urea and a bleaching agent' tion of urea to wood in the form of solutions, how-, ever, for the urea may also be applied to the wood surfaces in crystalline form, as a paste, or the like, the prime requisite being that the urea is applied in the broad range of 5 to 800 pounds per 1000 'boardfeet of wood, preferably from 10 to 200 pounds per 1000 board feet The quantities of bleaching agent employed may also be varied over X a fairly wide range, such as 1% to 20% by: weight, based on urea used, 2 to 15% being preferred.

Any of the well known bleaching agents-may be employed with urea to control the degree of bleaching or color of the final treated wood in'-;

eluding peroxides, such as sodium and hydrogen 3operoxides. Materials whichhave been found particularly effective, however, accordingto the present invention, are salts which in solution give ions ofbisulfite, hydros ulfite, chlorite, hypochlo rite, sulfoxylate, aldehyde sulfoxylate, such, specificially, as'sodium and potassium sulfite, sodium and ammonium bisulfite, sodium and potassium hydrosulfite, sodium chlorite,- sodium formalde hyde sulfoxylate, zinc formaldehyde sulfoxylate, andthe-like. l

As a further feature we have found that the addition of small quantitles of boric acid to the urea-bleaching agent mixture or solution has the effect of increasing the action or wood-lightening elfect by the bleaching agent. This effect of boric acid appears to be unique because we have found that wood treated with mixtures of urea bleaching agent and other acids is no lighter in color after seasoning than wood treated with straight urea and bleaching agents. The quantities of boric acid usedin any given instance may vary over a fairly wide range such as from 0.25 to 20% by weight based on the urea but we have found that the restricted range of proportions such as 1% to 5% is preferred.

The following examples will illustrate how the of the present mention;

invention may be practiced; parts being given by weight unless otherwise indicated.

Example 1 A green 1 /2 x 5 /2" x 8" Douglas fir sapwood block was surfaced 'andwet witlp tap'water; 1 A mixturesofi 100zparts'crystal urea and fi part's'sodium bisulfite was applied to the surface of the block at the rate of 42.5 pounds per 1000 board? 102i feet. A similar Douglas fir block was placed. on top of the urea-sodium bisulfite mixture and the two blocks were stored in a solid; pi1e:topermit solution of the urea and sodium bisulfiteim the.

water content of the wood.. After two days: the .7

blocks were placed in a kiln and dried at L70? and 50% relative humidity. Matched untreated 5 Nine pieces were used in each test.

blocks and blocks treated only'with 40 pounds of} I Douglasfir heartwood blocks were treated asin' Example 1' except that Zpalts; zinc formaldehydesulfoxyl'ate were usedinstead of 6: parts sodium bisulfite; Theurea zinc'formaldehyde sulfoxylate. treated block'swere bleached to a yellow color, whereas the urea treated blocks were slightly darker -brown than the untreated Douglas fir heartwood.

- "Example 3 A 2" x 12' if 3" partially air seasonedwliite oak board (moisture content about-"% )-was out into one foot lengths: One laloc'k'llvas painted with 40% urea; solution, one block was painted" with 40% ureasolution' containing 1% sodium bisul-* fite,'and'the-thirdib10ck was untreated. The'threeblocks were them kiln driedat' MO 'R- andrelative'liumidity: Thedrieduntreatedbloclrwas normal in color; theurea treatedblo'ck" was dark in color;whereas theairea-sodiumbisulfite treated block was" brighter in color than the" untreatedbloclr.

In" 'theabove example; thebleaohing effect ofthe bisulfit'e was confined to the surface: Any desired-depth of bleaching: and-degreeof bleach-- ingj canb'e' obtained by adjustingtheconcentra-- tiorr of'bl'eaohing agent andthe" lengt-lroftime the treated wood is left in solid pile; Increasing the length of time the woodisim solid pile-"increases" the" depth of bleaching; and increasing the eoncentration' of bleaching agent increases thedegree of bleaching; Great depth and degree'of bleaching were obtained in the following test.-

Example 4 Three separate kiln tests were carried out on random lots of 2" x 6" x 6 Douglas fir dimension stock containing mixed heartwood and sapwood. The kiln schedulefor eachtest was 2a.hours.at.160 F. and 801%? relative humidity,.22 /-:hours atZL'ZO'? F. and relative humidity, and22 /"2 hours at 180 F. and 40% relative humidity. Untreated wood was used in the first test. Wood treated with 40 pound oficrystalline urea per 1000 board feet and solid piled 2% dayswas used in the second test.

.Thefwoodi'usedin tlie third test was treated with 36. pounds' of, crystalline urea and 4 pounds of crystal. sodium bisulfite per 1000 board feet and solid piled 5% days. The wood used in the second ancttl'lird tests was wet with water before applyingthe crystalline chemicals.

The-dried, unsurfaced, urea-sodium bisulfite treated wood was much brighter in appearance than the untreated: st'ock.- The bleaching effect extended fii 'Tbelow the'surfaceof-thezwoodgwhich. is greater than the normal depth oi surfacing. The urea treated wood was? darker in appearance than'the-untreated;

Example 5 Douglas'firheartwood samples-1. a" 1514/ "x 2" were. soaked: five. minutes in an aqueous solution containing by weight 40%'- urea-,. 2% so'diumybi sulfite and 0.5% boric acid. After drying three days: in a: heated, room, the wood was lighters-in.- color than: untreated Douglasfir or. wood. soakedfiveiminuteszin .a'mixtureof 40 urea-and- 2% sodium bisulfit'e and dried-zas above;

In the following. claimslvunsea'sonedtwood means: wood containing. more: thanabout: 20%,- moisture on. thebasis. of the-ovendry weightof the-swoodi Various changes may be: made'in the-details andJpreferred embodiments of this invention w-ithoutdeparting therefrom orsacrificingt-head:-- vantages-thereof. Y

Weclaimtf 1'. In a: processi for: inhibiting oolorc'hangeyof:

- woodzduring;seasoningrthereof withmrea, the step:

which comprisescontacting; the said;- wood-with! a. composition: consisting of; parts by-' weight of: urea, 12 to 30 pa-rts of sodiunrzbisulfitee and-0:25

to: 20 parts of boric acid; the remainder -oflthe compositiombeing;wet-en. j 1 V 2.; In; a process; for. inhibiting. colonchangesot. wood. during seasoning; thereof. withx urea the:

Step which comprises contacting: the; said woodwith; a. composition; consistingyof. about 40%]:by-

weight of. urea, about:2-%.: sodium;bisulfite,-v and about 0.5r%= boricj: acid; .therremainderrof the: com-- positiorrbeingzwaten JQHN C.:W:0.0DHOHSE.. 

